Heat-expandable microspheres have a structure comprising a shell of thermoplastic resin and a blowing agent encapsulated therein, and are generally called heat-expandable microcapsules. Thermoplastic resins usually include vinylidene chloride copolymers, acrylonitrile copolymers, and acrylic copolymers; and a blowing agent mainly employed includes hydrocarbons, such as isobutane and isopentane. (Please see U.S. Pat. No. 3,615,972)
Heat-expandable microcapsules having high thermal durability, for example, those comprising a shell of thermoplastic resin produced from a component containing 80 weight percent or more of a nitrile monomer, 20 weight percent or less of a non-nitrile monomer, and a cross-linking agent are disclosed in JP A 62-286534. The method produces heat-expandable microcapsules in a process where a polymerizable mixture comprising a blowing agent, a polymerizable monomer, and a polymerization initiator is suspension-polymerized in an aqueous dispersing medium comprising colloidal silica as a dispersion stabilizer (a suspending agent), a diethanolamine-adipic acid condensate as a stabilizing auxiliary, and a polymerization auxiliary.
The polymerization auxiliary is usually used to control the generation of emulsion-polymerization products or prevent the generation of scale in an aqueous medium in the process of suspension-polymerization. Recently, a demand for developing a polymerization auxiliary, which improves the properties of resultant heat-expandable microcapsules and hollow particulates produced by thermally expanding the microcapsules, has been emerged, aside from the purpose for controlling the generation of emulsion-polymerization products and preventing scale in an aqueous medium. JP A 11-209504 discloses a method of producing heat-expandable microcapsules by applying so-called polymerization inhibitors, such as ascorbic acids and alkali metal nitrites, as a polymerization auxiliary instead of potassium dichromate. JP A 11-209504 describes heat-expandable microcapsules produced in the method sharply expand in heating and that are processed into uniformly expanded product (hollow particulates). However, the properties, such as expanding ratio, of the microcapsules are not sufficiently improved. Furthermore ascorbic acids are not preferable because they have poor thermal stability and decompose to lose their function as a polymerization inhibitor during polymerization reaction. In addition, alkali metal nitrites cause a problem, i.e., a cost for wastewater treatment after polymerization, because the Ordinance for Water Pollution Control Act which enforces the Water Pollution Control Act of Japan defines the limit values of alkali metal nitrites contained in ground water. Thus ascorbic acids and alkali metal nitrites are not satisfactory as polymerization auxiliaries at present, because they do not sufficiently improve the properties of resultant heat-expandable microcapsules and hollow particulates obtained by thermally expanding the microcapsules and they cause the problem mentioned above when they are employed in polymerization.
In the conventional research and development for hollow particulates obtained by thermally expanding heat-expandable microcapsules, researchers have concentrated on improving the properties of hollow particulates by studying the variants and ratio of thermoplastic resins and blowing agents constituting the hollow particulates.
Hollow particulates having improved durability against repeated-compression are described, for example, in JP A 2003-327482 and JP A 2003-327483, where thermally expanded microcapsules having polar groups on their surface are disclosed as hollow particulates which are durable against rupture in mixing and molding a ceramic composition. However, the durability of the microcapsules against rupture in mixing and molding has not been sufficiently improved.
JP A 2004-131361 describes hollow particulates for lightweight cement articles. The hollow particulates are produced by thermally expanding heat-expandable microcapsules which comprise a shell of polymer produced from a monomer composition and a cross-linking agent, and have an expanding ratio ranging from 20 times to 100 times. However, the durability of the hollow particulates has not been sufficiently improved.
Further, JP A 2005-067943 discloses a mixture of hollow particulates and heat-expandable microcapsules which are durable against rupture in mixing and molding a ceramic composition. However, the invention has only sharpened the particle size distribution of the mixture, and has not substantially improved the durability.
As mentioned above, hollow particulates having sufficiently improved durability against repeated compression have not been produced, though the durability of hollow particulates against repeated compression has been studied in various ways. The hollow particulates obtained by thermally expanding the heat-expandable microcapsules described in the Patent Reference 3 have also poor durability against repeated compression.